Abstract: An expert system having a processor which can receive input data and operate on it to produce data indicative of a parameter or parameters which are considered to define or represent the condition of equipment or a system to be diagnosed and which provides an adaptive pattern recognition facility which can be trained or instructed to recognize different combinations of input data as indicative of particular conditions of the equipment. The system is trained by coupling it initially to known good equipment into which definable faults are introduced. The adaptive pattern recognition facility has a series of sections with adjustable weight coefficients which are adjusted in accordance with a given algorithm as successive data combinations associated with the various faults are fed to the system. Once trained, the system can diagnose unknown equipment and provide outputs indicative of faults.